The present invention relates generally to digital protective relays for electrical distribution systems. More particularly, the present invention relates to a digital protective relay and system for providing enhanced protection and monitoring functions to an electrical distribution system.
Digital protective relays are well-known for monitoring and providing protective control of electrical power distribution systems. As used herein, the term xe2x80x9celectrical power distribution systemxe2x80x9d includes systems, and components of systems, for the generation, transmission, and/or distribution of electrical power to customers. Such relays are typically connected to a point on the electrical distribution system, monitor current, voltage, or other parameters, and provide protective control (e.g., by providing a control signal to cause power to be removed from the portion of the system with which the relay is associated) in the event that certain specified conditions occur. The functions of the protective relays can be enhanced in many ways. For example, digital protective relays include a microprocessor, which allows a user to determine how the protective relay will react under certain conditions.
Traditionally, protective relay logic has been relatively limited. Any unusual applications involving interlocks, blocking, or supervisory functions are typically achieved by hardwiring contact inputs and outputs. This requirement for auxiliary components and wiring make many logic schemes extremely difficult to implement.
Flexible logic schemes are known which allow a user to program logic functions within an individual relay. Such a relay typically includes fixed logic, which implements predetermined functions, and variable logic, which is controllable by the user to implement additional functions. In general, the relay incorporating programmable logic receives analog and digital inputs and uses these inputs to produce analog and digital outputs. If a relatively simple scheme is desired, where a contact input is used to block a measuring element of the relay, this selection is made when programming the measuring element. More complex logic schemes can be programmed in the individual relay. For example, if it is desired to have the closed state of one contact input and the operated state of a phase undervoltage element block the operation of a phase time overcurrent element, a logic equation is programmed to AND the two inputs to produce a virtual output, which is then selected when programming the phase time overcurrent element to be used as a blocking input.
While known programmable logic in a protective relay advantageously minimizes the requirement for auxiliary components and wiring, and makes more complex logic schemes possible, there are still limitations, as there are typically many protective relays operatively associated with an electrical distribution system.
It is known to communicate between protective relays. For example, U.S. Pat. No. 5,838,525 discloses high-speed single-pole trip logic for use in protective relays. The disclosed system includes a remote protective relay which generates a pilot signal to indicate the detection of a fault, and communicates the pilot signal to a local protective relay associated with a different location on an electrical line. The local protective relay uses the received pilot signal to supervise single-pole trip operations.
More complex communication schemes are also known. For example, the Electric Power Research Institute (EPRI) has specified a communication standard known as the UCA 2.0 xe2x80x9cGeneric Object Oriented Substation Eventxe2x80x9d (GOOSE) specification. GOOSE is based on the asynchronous reporting of a relay""s digital output status to other peer devices on a network. GOOSE messages are designed to be short, high priority, and communicated with a high level of reliability. To achieve reliability, GOOSE messages are repeated as long as they are valid. GOOSE messages need not be acknowledged and so may be multicast. The GOOSE message structure contains space for 128 bit pairs representing digital point state information. The GOOSE specification provides 32 xe2x80x9cDNAxe2x80x9d bit pairs, which are status bits representing pre-defined events. All remaining bit pairs are xe2x80x9cUserStxe2x80x9d bit pairs, which are status bits representing user-defined events. The UCA 2.0 specification includes features that are used to cope with the loss of communication between transmitting and receiving devices. Each transmitting device sends a GOOSE message upon a successful powerup, when the state of any included point changes, or after a specified interval (a xe2x80x9cdefault updatexe2x80x9d time) if a change of state has not occurred. The transmitting device also sends a xe2x80x9chold timexe2x80x9d which is set to three times the programmed default time.
Where protective relays are provided with communications capabilities, control logic is typically predefined, pre-programmed, or involves establishing a master-slave relationship between communicating protective devices. Implementing control logic can also require relatively complex wiring schemes between relays. Such protective relays and schemes are undesirable in that they limit or make it difficult to provide a wide variety of protection schemes.
It would be desirable for a protective relay to incorporate programmable control logic, and also to be able to communicate outputs of the programmable control logic to remote protective relays associated with the same electrical distribution system, in order to provide enhanced protection and control. It would also be desirable to provide enhanced control and configuration options for providing enhanced protective control of electrical distribution systems, and to be able to implement more complex logic schemes than are possible by programming logic in each individual protective relay associated with a network. However, known protective relays and protection schemes are unsatisfactory for this purpose.
The present invention overcomes the disadvantages of prior solutions, and achieves additional advantages, by providing for a power control and monitoring system for an electrical power distribution network which includes a plurality of digital protective relays which include programmable logic and which can communicate control logic inputs and outputs over a peer-to-peer communications network. Each digital protective relay includes facilities, terminals, devices or equipment for connecting to an electrical distribution system and at least one communications port for connection to the peer-to-peer communications network. The peer-to-peer communications network is preferably an ethernet network.